First of all, don't approach Dallis and her group at the gate when you start the game, take the side ladder up the wall and do every quest/exp possible in the fort to level up without actually escaping the fort into the swamps (that despawns her party even if you don't speak with them). As long as you don't approach their party or enter the swamps, Dallis and her crew stand idle at the gate waiting forever. I made it to level 5 from the stuff in the fort (Tactician mode).
Once leveled, climb the wall to the side of Dallis's party and use Telekinesis with huge crates to block off the top of both ladders up the wall. Put your entire party on the wall above Dallis's group, but out of line of sight from the ground. Start combat by shooting an arcing projectile into their group (fossile strike works). The enemies will try to get to you, but because a crate is blocking the upper part of the ladder, they can't actually get up the wall. One of the magisters has a crossbow, and he will shoot the crate from the ground, trying to break it. Have one of your party walk to the wall's edge, teleport that magister up into your group on the wall, then move back away from the wall's edge during the same turn. If you end your turn in line of sight of the guys on the ground, it's over because the Pet's will blink up with their backstabs and Alexander will netherswap people down.
Finish off the crossbow magister, then start bombing the guys on the ground by using 1 AP to approach the walls edge, cast a few spells, then your last AP to end your turn out of LOS from the ground. It's slow, but apparently none of them know any healing spells, so you win eventually. Once Alexander or Dallis get to about half life, they both retreat from combat and give you the achievement (and Dallis's unique hammer). The magisters and the 2 Geist pets remain, and you can just clean them up with the same hit and run tactics from out of LOS.
To speed it up some I teleported the Geists up as well so my melee could help finish them, but they must be CC'ed at all times or their backstabs will one shot your low level team at that point in the game (on Tactician at least).